One way to define poor color vision is the inability to discriminate between different hues. There are different varieties and degrees of poor color vision. The “normal” human retina contains three cone photoreceptors termed the long wavelength sensitive, middle wavelength sensitive, and short wavelength sensitive cones. Protan deficiencies refer to either a loss or an abnormal absorption of the long wavelength sensitive cones, deutan deficiencies refer to either a loss or an abnormal absorption of the middle wavelength sensitive cones, and Tritan deficiencies refer to either a loss or an abnormal absorption of the short wavelength sensitive cones. The incidence of protan and deutan deficiencies are common in congenital color problems and the incidence of tritan and more diffuse deficiencies are more common with acquired color deficiencies. Poor color vision is a relatively common problem for many subjects. Clinical estimates suggest the number of subjects having a congenital color vision in the population is nearly 8% for males and 1% for females. The incidence is much higher if all of the other known causes of color deficiencies such as diseases, side-effects of some medications, trauma, aging, and exposure to certain chemicals or other environmental factors are included. Poor color vision is generally undesirable because many jobs and everyday activities requires average or superior color discrimination.
Several tests for determining color discrimination are widely used. These tests are broadly categorized into pseudoisochromatic plates, arrangement tests, matching tests, and naming tests. Arrangement tests are generally considered reliable and are frequently used by practioners. In arrangement tests, subjects sequentially arrange, according to perceived color proximity, a number of loose caps that vary in hue. Three well-known arrangement tests are the Farnsworth Munsell D-15, the L'Anthony desaturated D-15, and the Farnsworth Munsell 100 hue test.
The Farnsworth Munsell 100 hue test is designed to detect all types of color vision abnormalities. Additionally, the test can separate subjects with normal color vision into classes of superior, average and low color discrimination and detect and measure the zones of color confusion. The test uses 85 movable and 8 fixed caps, each with a diameter of 21 mm. These 93 caps, each having a color spot, form a sample of the natural color spectrum and of the range of purple colors. The caps are distributed among four trays. When placed one abutting the other using the criterion that the most similar colors are arranged one after the other, the caps produce a closed color circle. Each cap, and hence each color spot, is assigned a number which makes it possible to calculate how many partial mistakes are made and to calculate the total number of errors. The pattern and total number of errors are parameters used to monitor the type and severity of color vision disorders.
There are two important reliability issues associated with the implementation and scoring process of the three cap arrangement tests. The first issue relates to the transferring of the subject's arrangement into a particular scoring sheet. Conventionally, the tester does this by lifting the caps arranged by the subject, flipping them upside down, reading the number on the underside of the cap, and transcribing the number in the particular scoring sheet. This process is time-consuming and involves a certain likelihood of error resulting in inaccurate reports. The second issue relates to the data analysis conducted on the discrepancies between the subject-arranged cap arrangement and the correct color order. Where this scoring is performed manually is it time consuming, extending up to an hour.
Currently there are several computer-based methods that expedite the scoring calculations and interpretation of test results. However, these methods still require the arranged caps to be manually lifted and flipped upside down and manually entered into the program for analysis. To eliminate these shortcomings, an automatic transcription of the subject's arrangements to the host computer is needed.
U.S. Pat. No. 5,938,620 attempts to fulfill these objectives with an apparatus including a positioning tray, a plurality of color caps movably located in the positioning tray, and a series of fixed magnets located on the bottom of each of the color caps. Each cap has a unique arrangement of magnets. A series of magnetic detectors in the tray identify the strength and arrangement of the magnets. An evaluation unit analyzes the cap order and the information is then transmitted for further evaluation to a computer system. This invention requires one detector for each magnet located on the bottom of each cap. The number of magnets needed per cap depends on the number of caps used for the testing. Because several detectors are needed per cap in order to administer the arrangement tests discussed above, the resulting invention can be quite costly. Another shortcoming of this invention relates to the detection accuracy. A particular cap must be oriented so the magnets located on the cap align with the detectors corresponding to each cap in the positioning tray to ensure an accurate detection of the particular cap. Additionally, there is a certain likelihood of cross-talk between the caps and detectors when the caps are in certain arrangements. A further shortcoming involves the cost and monitoring required to replace or recalibrate the invention because the magnets have lost strength over the course of time or have been dropped or otherwise damaged in the normal course of testing. Thus, there remains a need for a device for determining color discrimination of a subject by automatically detecting the sequence of caps which is efficient, reliable, robust, and relatively inexpensive.